The present invention generally relates to a spark plug, and more particularly to a spark plug used for an internal combustion engine, which includes noble metallic tips bonded to a center electrode and a ground electrode so as to possess excellent ignitability and is capable of improving exhaustion resistance of these noble metallic tips even when subjected to severe thermal loads.
To assure excellent ignitability, U.S. Pat. No. 4,109,633 discloses a spark plug equipped with center and ground electrodes being configured into a slender structure so as to protrude from electrode support portions.
Furthermore, to assure exhaustion resistance for the electrodes, there is a conventional slender structure which includes noble metallic tips made of Pt, Pd, Au, or their alloys which are fixed to opposed surfaces of the center and ground electrodes.
However, from the recent trends of higher power output, low fuel consumption, and low exhaust gas emissions, the spark plugs of the engines are forced to be exposed to high-temperature combustion environment and, accordingly, the electrode temperature of the spark plug extremely increases.
Furthermore, when a ground electrode employs a slender structure including a protruding noble metallic tip, this noble metallic tip tends to become a heat spot where the exhaustion of tip material accelerates. Hence, the lifetime of spark plug becomes very short.
In view of the foregoing problems of the prior art, the present invention has an object to provide a spark plug including noble metallic tips fixed to opposed surfaces of center and ground electrodes disposed in an opposed relationship via a discharge gap and capable of assuring excellent exhaustion resistance of these noble metallic tips.
The inventor of this application has earnestly conducted research and development to overcome the foregoing problems. In general, the exhaustion of a noble metallic tip can be regarded as co-occurrence of spark exhaustion resulting from melting or fusion of a noble metallic tip caused by discharge energy, and oxidative and volatile exhaustion resulting from oxidation and volatilization of a noble metallic tip in high-temperature environments. When compared, the center electrode and the ground electrode are different from each other in the exhaustion mechanism of their noble metallic tips. Thus, there is a significantly difference between the center electrode and the ground electrode in the ratio of the spark exhaustion to the oxidative and volatile exhaustion. Regarding the mechanism of the oxidative and volatile exhaustion, the oxidative and volatile exhaustion is believed to occur in a high-temperature atmosphere in such a manner that an oxide film is first formed on a tip surface and then the oxide film falls off the noble metallic tip.
In general, the noble metallic tip of a center electrode has minus polarity, and accordingly the percentage of spark exhaustion is relatively large while the percentage of oxidative and volatile exhaustion is small. On the other hand, the noble metallic tip of a ground electrode has a higher temperature compared with the noble metallic tip of a center electrode. Accordingly, the percentage of oxidative and volatile exhaustion is large. The percentage of spark exhaustion is relatively small because the noble metallic tip of a ground electrode has plus polarity.
Considering the above facts (i.e., the differences in their exhaustion mechanisms), the inventor have experimentally optimized the material composition for each of the noble metallic tip of a center electrode and the noble metallic tip of a ground electrode.
More specifically, in order to accomplish the above and other related objects, the present invention provides a first spark plug including a center electrode, a ground electrode disposed in an opposed relationship with the center electrode via a discharge gap, a noble metallic tip fixed to an opposed surface of the center electrode, and a noble metallic tip fixed to an opposed surface of the ground electrode, wherein the noble metallic tip of the ground electrode protrudes from the opposed surface of the ground electrode by a protrusion amount xe2x80x98txe2x80x99 not less than 0.3 mm, and the noble metallic tip of the ground electrode possesses excellent oxidative and volatile resistance compared with the noble metallic tip of the center electrode.
Preferably, an oxidized and volatilized ratio X, being defined as a ratio Lmax2/Lmax1, is not greater than 0.8 (i.e., Xxe2x89xa60.8), where Lmax1 represents a maximum oxidized and volatilized width of the noble metallic tip of the center electrode and Lmax2 represents a maximum oxidized and volatilized width of the noble metallic tip of the ground electrode observed after the noble metallic tip of the center electrode and the noble metallic tip of the ground electrode are left in the air for 30 hours at the temperature of 1,100xc2x0 C.
Furthermore, it is preferable that the noble metallic tip of the ground electrode has a cross-sectional area xe2x80x98Axe2x80x99 not less than 0.1 mm2 and not greater than 1.15 mm2, and the protrusion amount xe2x80x98txe2x80x99 is not greater than 1.5 mm.
Furthermore, to improve the oxidative and volatile resistance of the noble metallic tip of the ground electrode, the inventor has come across an idea of adopting a noble metallic tip containing an additive which is so easily oxidized to form an oxide film covering or protecting the surface of the noble metallic tip.
Hence, the inventor has conducted endurance tests by varying the content of the additive to check the influence of additive given to the exhaustion resistance of a noble metallic tip made of an Ir alloy which has higher melting point. Based on such experimental research, the invention proposes to employ the following spark plug arrangement.
More specifically, the present invention provides a second spark plug including a center electrode, a ground electrode disposed in an opposed relationship with the center electrode via a discharge gap, a noble metallic tip fixed to an opposed surface of the center electrode, and a noble metallic tip fixed to an opposed surface of the ground electrode, wherein the noble metallic tip of the ground electrode protrudes from the opposed surface of the ground electrode by a protrusion amount xe2x80x98txe2x80x99 not less than 0.3 mm, each of the noble metallic tip of the center electrode and the noble metallic tip of the ground electrode is made of an iridium alloy which contains iridium whose content exceeds 50% by weight and also contains at least one kind of additive, and a total amount of all additives contained in the noble metallic tip of the ground electrode is not less than 15% by weight.
Furthermore, the inventor has conducted endurance tests to optimize the weight percentage of all additives contained in the noble metallic tip of the ground electrode. Based on such experimental research, the invention proposes to employ the following spark plug arrangement.
Namely, according to the second spark plug of the present invention, it is preferable that the total weight percentage of all additives contained in the noble metallic tip of the ground electrode is 1.5 times or more a total weight percentage of all additives contained in the noble metallic tip of the center electrode.
Furthermore, the present invention provides a third spark plug including a center electrode, a ground electrode disposed in an opposed relationship with the center electrode via a discharge gap, a noble metallic tip fixed to an opposed surface of the center electrode, and a noble metallic tip fixed to an opposed surface of the ground electrode, wherein the noble metallic tip of the ground electrode protrudes from the opposed surface of the ground electrode by a protrusion amount xe2x80x98txe2x80x99 not less than 0.3 mm, and the noble metallic tip of the center electrode is made of an iridium alloy which contains iridium whose content exceeds 50% by weight and also contains at least one kind of additive, and the noble metallic tip of the ground electrode is made of a platinum alloy which contains platinum whose content exceeds 50% by weight and also contains at least one kind of additive.
Furthermore, according to the second or third spark plug of the present invention, it is preferable that the noble metallic tip of the ground electrode has a cross-sectional area xe2x80x98Axe2x80x99 not less than 0.1 mm2 and not greater than 1.15 mm2, and the protrusion amount xe2x80x98txe2x80x99 is not greater than 1.5 mm.
Furthermore, according to the second or third spark plug of the present invention, it is preferable that the additive contained in the noble metallic tips of the center electrode and the ground electrode is selected from the group consisting of Ir (iridium), Pt (platinum), Rh (rhodium), Ni (nickel), W (tungsten), Pd (palladium), Ru (ruthenium), Os (osmium), Al (aluminum), Y (yttrium), Y2O3 (yttrium oxide), and Re (rhenium).
Furthermore, according to the third spark plug of the present invention, it is preferable that all additives contained in the noble metallic tip of the ground electrode has a melting point higher than that of Pt.
Furthermore, according to the third spark plug of the present invention, it is preferable that all additives contained in the noble metallic tip of the ground electrode has a linear expansion coefficient smaller than that of Pt, and the noble metallic tip of the ground electrode is fixed to the opposed surface of the ground electrode by laser welding.